How to save blender project without the original files [Unable to pack file Warning]
The .blend in your link comes with a file missing (Fields_wallpapers_488.jpg).
It was not packed properly by the creator of the file.

Packed files do not need to be unpacked for them to work in Blender. If files are unpacked they are usually used from the location they are unpacked to and automatic packing is turned off. If it's enabled again then the files are packed from the location they were unpacked. As you can see asset packing and unpacking functionality is quite complete in Blender and it usually works as intended. In this specific case the file comes with a missing asset - that's why it's not possible to find. The creator of the file must have received the same warning and chose to ignore it.